Steel wire material and method for manufacturing same

ABSTRACT

The steel wire material of the present invention contains 0.05 to 1.2% of C (mass %; same for the chemical components hereafter), 0.01 to 0.5% of Si, 0.1 to 1.5% of Mn, 0.02% or less (but not 0%) of P, 0.02% or less (but not 0%) of S, and 0.005% or less (but not 0%) of N, with the balance being iron and inevitable impurities. The wire material has a scale layer that is no thicker than 7.0 μm or less. The scale layer has an FeO percentage of 30 to 80 vol % and an Fe 2 SiO 4  percentage of less than 0.1 vol %. The scale layer that is formed will not peel when cooled after hot rolling or during storage and transport, but will easily peel during mechanical descaling.

TECHNICAL FIELD

The present invention relates to a steel wire material and a method formanufacturing the same, and relates more specifically to a hot rolledsteel wire material (hereinafter simply referred to as “wire material”)formed with a thin scale not peeling off during cooling after hotrolling and at the time of storage and transportation and easilyremovable by mechanical descaling, and a method for manufacturing thesame.

BACKGROUND ART

A scale is formed normally on the surface of a wire materialmanufactured by hot rolling, and it is required to remove the scalebefore subjecting the wire material to secondary work such as drawingand the like. As such a scale removing method before secondary work, abatch type acid cleaning method was employed in prior arts, however, inrecent years, from the viewpoints of the environmental pollution andcost reduction, a mechanical descaling (hereinafter referred to as MD)method has come to be employed. Therefore, the wire material is requiredto be formed with a scale with excellent MD performance.

As methods for manufacturing a wire material formed with a scale withexcellent MD performance, Patent Literatures 1-5 can be cited forexample. In Patent Literatures 1, 2, the scale amount remaining in thewire material after MD is reduced by forming a scale which is high inFeO ratio and thick. In Patent Literature 3, by lowering the boundaryface roughness, propagation of the crack occurring on the boundary faceof the scale is promoted, and the remaining scale amount is reduced. InPatent Literatures 4, 5, by controlling the area ratio of the holesinside the scale, the peeling performance of the scale is improved.

However, Patent Literatures 1-5 described above have problems asdescribed below. According to the method of forming the scale thick asPatent Literatures 1, 2, drop of the yield is caused, the scale peelsoff during the cooling step and at the time of storage andtransportation, and the rust is generated. Also, when the scale isthick, even when a bending strain is applied to the wire material by theMD method and the wire material surface is subjected to brushing, it isdifficult to perfectly remove the scale. More specifically, according tothe MD method, unlike the batch type acid cleaning method, it isdifficult to remove the entire scale evenly and stably, and even whenthe wire material formed with thick scale is subjected to MD, thesurface of the wire material may occasionally be spotted with finelycrushed scale powder. When the remaining scale remaining locally thusincreases, in the secondary work such as drawing and the like, problemssuch as occurrence of a flaw due to the defective lubrication, loweringof the lifetime of the dice and the like are caused.

Also, it is difficult to stably lower the boundary face roughness by themethod of lowering the boundary face roughness such as Patent Literature3, it is difficult to stably form the holes even by the method offorming holes inside the scale such as Patent Literatures 4, 5, and itis difficult to stably reduce the remaining scale amount according toeither of these technologies.

Further, in these Patent Literatures 1-5, peeling off of the scale dueto the compression stress generated during cooling is not considered atall, and there was a problem that the rust was generated in the wirematerial before MD by peeling off of the scale during cooling and at thetime of storage and transportation.

CITATION LIST Patent Literature

-   [Patent Literature 1] Japanese Unexamined Patent Application    Publication No. H4-293721-   [Patent Literature 2] Japanese Unexamined Patent Application    Publication No. H11-172332-   [Patent Literature 3] Japanese Unexamined Patent Application    Publication No. H8-295992-   [Patent Literature 4] Japanese Unexamined Patent Application    Publication No. H10-324923-   [Patent Literature 5] Japanese Unexamined Patent Application    Publication No. 2006-28619

SUMMARY OF INVENTION Technical Problems

The present invention has been developed in view of the circumstancesdescribed above, and its object is to provide a wire material formedwith a scale not peeling off during cooling after hot rolling and at thetime of storage and transportation and easily peeling off at the time ofMD, and a method for manufacturing the same.

Solution to Problems

The steel wire material of the present invention which solved theproblems described above is a steel wire material containing C:0.05-1.2% (“%” means “% by mass”, hereinafter the same for chemicalcomponents), Si: 0.01-0.5%, Mn: 0.1-1.5%, P: 0.02% or less (notincluding 0%), S: 0.02% or less (not including 0%), and N: 0.005% orless (not including 0%), with the remainder being iron and unavoidableimpurities, in which a scale with 7.0 μm or less thickness is included,FeO ratio inside the scale is 30-80 vol %, and Fe₂SiO₄ ratio is lessthan 0.1 vol %.

According to the necessity, the steel wire material of the presentinvention may also contain (a) Cr: 0.3% or less (not including 0%)and/or Ni: 0.3% or less (not including 0%), (b) Cu: 0.2% or less (notincluding 0%), (c) at least one element selected from a group consistingof Nb, V, Ti, Hf and Zr by 0.1% or less (not including 0%) in total, (d)Al: 0.1% or less (not including 0%), (e) B: 0.005% or less (notincluding 0%), and (f) Ca: 0.01% or less (not including 0%) and/or Mg:0.01% or less (not including 0%).

Further, the present invention also includes a method for manufacturinga steel wire material including a step of hot rolling steel of any ofthe chemical compositions described above, a step of thereafter windingup the hot rolled steel at 750-880° C., and a step of cooling the woundsteel while injecting a gas mixture of oxygen and an inert gas whoseoxygen fraction is less than 20 vol % or an inert gas. It is preferablethat the inert gas is nitrogen.

Advantageous Effects of Invention

In the steel wire material of the present invention, the FeO ratio isappropriately controlled to a predetermined range (30-80 vol %), and athin (7 μm or less) scale is included. Accordingly, the scale does notpeel off during cooling after hot rolling and at the time of storage andtransportation, and generation of the rust can be prevented. Further,according to the present invention, because the scale easily peels offat the time of MD, sufficient peeling performance can be secured with asimple descaling device, adverse effects (a flaw on the surface of thewire material, defective lubrication and the like due to leaving thescale unremoved) are not exerted in secondary work such as drawing andthe like, and the steel wire material of high quality can be provided.Also, because the scale loss is less, high yield can be maintained.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a graph showing the relation between the FeO ratio inside thescale and the remaining scale area ratio after MD.

FIG. 2 is a graph showing the relation between the scale thickness andthe scale peeling ratio of the rolled material.

DESCRIPTION OF EMBODIMENTS

In a cooling step during a manufacturing process of a wire material,normally, a compression stress is generated inside the scale due to thedifference in the coefficient of thermal expansion between the base ironand the scale. As a result, the scale naturally peels off during thecooling step or at the time of storage and transportation of the wirematerial thereafter, which became the cause of generation of the rust.Also, the scale is removed by MD before executing secondary work such asdrawing and the like, and the lifetime of the dice is shortened when thescale remains after MD. Therefore, the wire material having a scale thatdoes not peel off in the cooling step during the manufacturing processand at the time of storage and transportation and easily peels off atthe time of MD has been desired.

The MD method is a method for making the scale peel off by applyingstrain to the wire material to generate cracks inside the scale or inthe boundary face of the base iron and the scale. Conventionally,increase of the FeO ratio inside the scale has been executed in order toimprove the peeling performance of the scale. This is because theincrease of the FeO ratio inside the scale is considered to be effectivein improving the peeling performance of the scale at the time of MDbecause the strength of FeO is weaker than Fe₂O₃ and Fe₃O₄. In order toincrease the FeO ratio inside the scale, it is normally required to forma scale (a secondary scale formed in or after descaling before finishrolling) at a high temperature, however there was a problem that, whenthe scale was formed at a high temperature, the thickness of the scaleincreased, the scale loss increased, and the thick scale peeled offduring the cooling step and at the time of storage and transportation.In other words, it was extremely difficult to make the thickness of thescale thin and to secure the FeO ratio inside the scale.

So, as a result of studies by the present inventors, it was found outthat, when the winding temperature after the hot rolling was madecomparatively low temperature and cooling was thereafter executed whileinjecting a gas mixture of oxygen and an inert gas whose oxygen fractionwas comparatively low or an inert gas, the scale could be made thin andthe FeO ratio inside the scale could be secured by a predetermined ratioor more.

When the thickness of the scale was studied in more detail, it was foundout that, if the thickness of the scale was 7.0 μm or less, adhesivenessagainst the base iron was excellent, and the scale did not peel off inthe middle of cooling and at the time of storage and transportation. Thescale thickness is preferably 6.5 μm or less, more preferably 6.0 μm orless (particularly 5.5 μm or less). Although the lower limit of thescale thickness is not particularly limited, it is approximately 0.9 μmnormally.

Further, the present inventors investigated the relation between the FeOratio inside the scale and the MD performance. More specifically, thewire material with 200 mm length having a composition of 0.9% C-0.25%Si-0.86% Mn-0.007% P-0.0063% S-0.002% N was used, the windingtemperature condition was changed, and the samples whose composition ofthe scale was adjusted were manufactured. Also, the winding temperaturewas changed in the range of 700-1,000° C., and N₂-10 vol % O₂ gas wasused for cooling after winding. The scale was made peel off by applyinga deformation strain (6%) equivalent to MD to the manufactured sample,and the scale amount (area ratio) remained was measured by imageanalysis similarly to the example described below. FIG. 1 is a graphshowing the relation between the FeO ratio inside the scale and the arearatio of the scale that remained after MD.

According to FIG. 1, it is known that, when the FeO ratio inside thescale is 30-80 vol %, the remaining scale amount after MD can be reducedsufficiently. The FeO ratio is preferably 35 vol % or more and 75 vol %or less, more preferably 40 vol % or more and 70 vol % or less, andfurther more preferably 45 vol % or more and 65 vol % or less.

Also, the Fe₂SiO₄ (fayalite) ratio inside the scale is to be less than0.1 vol %. When excessively formed, Fe₂SiO₄ is formed unevenly on theboundary face between the scale and the base iron, the scale unevenlypeels off at the time of MD, and therefore the MD performancedeteriorates. The Fe₂SiO₄ ratio is preferably 0.09 vol % or less, morepreferably 0.08 vol % or less, and further more preferably 0.07 vol % orless. On the other hand, since Fe₂SiO₄ inside the scale is an oxide thatis brittle and easily peels off and is formed evenly and thin if itsamount is slight, it has an action of improving the MD performance. Inorder to exert such action effectively, it is preferable to secureFe₂SiO₄ by 0.01 vol % or more, more preferably 0.02 vol % or more, andfurther more preferably 0.03 vol % or more.

In the scale in the present invention, Fe₂O₃, Fe₃O₄ and the like areincluded other than FeO and Fe₂SiO₄.

By making the thickness of the scale and the area ratio of the fineholes as described above, the remaining scale amount after MD can bemade 30% or less by the area ratio relative to the scale amount beforeMD. This is equivalent to approximately 0.05 mass % or less in terms ofthe remaining scale amount relative to the mass of the steel wirematerial. The remaining scale amount is preferably 25% or less by area,more preferably 20% or less by area.

In order to form the scale described above, it is important to hot-rollthe steel with the chemical composition described below, to thereafterexecute winding at a comparatively low temperature (750-880° C.), andthen to execute cooling while injecting a gas mixture of oxygen and aninert gas whose oxygen fraction is low or an inert gas. By executingwinding at a low temperature, the scale can be made thin. Also, byinjecting the gas whose oxygen fraction is low or not including oxygenas described above and executing cooling, FeO can be secured by apredetermined amount or more without converting FeO formed to Fe₃O₄.

When the winding temperature after hot rolling exceeds 880° C., thescale thickness exceeds 7.0 μm, the FeO ratio inside the scale exceeds80 vol %, and the MD performance deteriorates. Also, when the windingtemperature exceeds 880° C., Fe₂SiO₄ (fayalite) possibly exceeds 0.1 vol% and is formed unevenly on the boundary surface between the scale andthe base iron, the scale peels off unevenly at the time of MD, and theMD performance deteriorates. On the other hand, when the windingtemperature is below 750° C., 30 vol % or more of the FeO ratio cannotbe secured, and the MD performance deteriorates. The winding temperatureis preferably 770° C. or above and 875° C. or below, more preferably790° C. or above and 860° C. or below.

Cooling after hot rolling is executed while injecting a gas mixture ofoxygen and an inert gas whose oxygen fraction is less than 20 vol % oran inert gas. By cooling while injecting such a gas with low oxygenfraction or not containing oxygen, FeO already formed can be preventedfrom being converted to Fe₃O₄, and the FeO ratio inside the scale can besecured. The oxygen fraction is preferably 10 vol % or less, morepreferably 5 vol % or less, and further more preferably 0 vol % (thatis, the inert gas only). Argon, nitrogen and the like can be cited asthe inert gas, and nitrogen is preferable. Although the cooling stoppingtemperature in cooling executed while injecting the gas described aboveis not particularly limited, cooling may be executed to approximately550-650° C. for example while injecting the gas described above, andcooling may be executed thereafter to the room temperature in theatmospheric air.

Below, the chemical composition of the steel wire material of thepresent invention will be described.

C: 0.05-1.2%

C is an element greatly affecting the mechanical properties of steel. Inorder to secure the strength of the wire material, the C amount wasstipulated to be 0.05% or more. The C amount is preferably 0.15% ormore, more preferably 0.3% or more. On the other hand, when the C amountis excessively high, the hot workability in manufacturing the wirematerial deteriorates. Therefore, the C amount was stipulated to be 1.2%or less. The C amount is preferably 1.1% or less, more preferably 1.0%or less.

Si: 0.01-0.5%

Si is an element required for deoxidizing steel. When its content is toolow, formation of Fe₂SiO₄ (fayalite) becomes insufficient, and the MDperformance deteriorates. Therefore, the Si amount was stipulated to be0.01% or more. The Si amount is preferably 0.1% or more, more preferably0.2% or more. On the other hand, when the Si amount is excessively high,by excessive formation of Fe₂SiO₄ (fayalite), such problems occur thatthe MD performance extremely deteriorates, a surface decarburized layeris formed, and the like. Therefore, the Si amount was stipulated to be0.5% or less. The Si amount is preferably 0.45% or less, more preferably0.4% or less.

Mn: 0.1-1.5%

Mn is an element useful in securing the quenchability of steel andincreasing the strength. In order to effectively exert such actions, theMn amount was stipulated to be 0.1% or more. The Mn amount is preferably0.2% or more, more preferably 0.4% or more. On the other hand, when theMn amount is excessively high, segregation occurs in the cooling stepafter the hot rolling, and super-cooled structure (martensite and thelike) harmful for the drawability and the like is liable to begenerated. Therefore, the Mn amount was stipulated to be 1.5% or less.The Mn amount is preferably 1.4% or less, more preferably 1.2% or less.

P: 0.02% or Less (not Including 0%)

P is an element deteriorating the toughness and ductility of steel. Inorder to prevent the wire breakage in the drawing step and the like, theP amount was stipulated to be 0.02% or less. The P amount is preferably0.01% or less, more preferably 0.005% or less. Although the lower limitof the P amount is not particularly limited, it is approximately 0.001%normally.

S: 0.02% or Less (not Including 0%)

Similarly to P, S is an element deteriorating the toughness andductility of steel. In order to prevent the wire breakage in the drawingstep and the twisting step thereafter, the S amount was stipulated to be0.02% or less. The S amount is preferably 0.01% or less, more preferably0.005% or less. Although the lower limit of the S amount is notparticularly limited, it is approximately 0.001% normally.

N: 0.005% or Less (not Including 0%)

N is an element deteriorating the ductility of steel when the contentthereof becomes excessively high. Therefore, the N amount was stipulatedto be 0.005% or less. The N amount is preferably 0.004% or less, morepreferably 0.003% or less. Although the lower limit of the N amount isnot particularly limited, it is approximately 0.001% normally.

The fundamental composition of the steel wire material of the presentinvention is as described above, and the balance is substantially iron.However, inclusion of unavoidable impurities brought in due tosituations of raw materials, materials, manufacturing facilities and thelike in the steel wire material is allowed as a matter of course.Further, it is also recommended to add elements described belowaccording to the necessity within a range not impeding the actions andeffects of the present invention.

Cr: 0.3% or Less (not Including 0%) and/or Ni: 0.3% or Less (notIncluding 0%)

Both of Cr and Ni are elements enhancing the quenchability of steel andcontributing to increase the strength. In order to exert such actionseffectively, the Cr amount is preferably 0.05% or more and the Ni amountis preferably 0.03% or more. Both of the Cr amount and Ni amount aremore preferably 0.10% or more, and further more preferably 0.12% ormore. On the other hand, when the Cr amount and Ni amount areexcessively high, the martensite structure is liable to be generated,adhesiveness of the scale and the base iron increases excessively high,and the peeling performance of the scale at the time of MD deteriorates.Therefore, both of the Cr amount and Ni amount are preferably 0.3% orless, more preferably 0.25% or less, and further more preferably 0.20%or less.

Cu: 0.2% or Less (not Including 0%)

Cu is an element having an action of promoting peeling of the scale. Inorder to exert such action effectively, the Cu amount is preferably0.01% or more, more preferably 0.05% or more, and further morepreferably 0.10% or more. On the other hand, when the Cu amount isexcessively high, peeling of the scale is promoted excessively, thescale peels off during rolling, other scales which are thin and highlyadhesive are generated on the peeled surface, and the rust is generatedwhen the wire material coil is stored and transported. Therefore, the Cuamount is preferably 0.2% or less, more preferably 0.17% or less, andfurther more preferably 0.15% or less.

At Least One Element Selected from a Group Consisting of Nb, V, Ti, Hfand Zr: 0.1% Or Less (not Including 0%) in Total

All of Nb, V, Ti, Hf and Zr are elements forming fine carbonitride andcontributing to increase the strength. In order to exert such actionseffectively, all of the Nb amount, V amount, Ti amount, Hf amount and Zramount are preferably 0.003% or more, more preferably 0.007% or more,and further more preferably 0.01% or more. On the other hand, when theseelements are excessively high, the ductility deteriorates, and thereforethe total amount thereof is preferably 0.1% or less, more preferably0.08% or less, and further more preferably 0.06% or less.

Al: 0.1% or Less (not Including 0%)

Al is an element effective as a deoxidizing agent. In order to exertsuch action effectively, the Al amount is preferably 0.001% or more,more preferably 0.005% or more, and further more preferably 0.01% ormore. On the other hand, when the Al amount is excessively high,oxide-based inclusions such as Al₂O₃ and the like increase, and wirebreakage frequently occurs in drawing work and the like. Therefore, theAl amount is preferably 0.1% or less, more preferably 0.08% or less, andfurther more preferably 0.06% or less.

B: 0.005% or Less (not Including 0%)

B is an element suppressing formation of ferrite by being present asfree B (B that does not form the compound) solid-solved in steel, and isan element effective particularly in a high strength wire material whichrequires suppression of a longitudinal crack. In order to exert suchactions effectively, the B amount is preferably 0.0001% or more, morepreferably 0.0005% or more, and further more preferably 0.0010% or more.On the other hand, when the B amount is excessively high, the ductilitydeteriorates. Therefore, the B amount is preferably 0.005% or less, morepreferably 0.0040% or less, and further more preferably 0.0035% or less.

Ca: 0.01% or Less (not Including 0%) and/or Mg: 0.01% or Less (notIncluding 0%)

Both of Ca and Mg are elements having an action of controlling the formof the inclusions and enhancing the ductility. Further, Ca also has anaction of enhancing the corrosion resistance of the steel material. Inorder to exert such actions effectively, both of the Ca amount and theMg amount are preferably 0.001% or more, more preferably 0.002% or more,and further more preferably 0.003% or more. On the other hand, whenthese elements are excessively high, the workability deteriorates.Therefore, both of the Ca amount and the Mg amount are preferably 0.01%or less, more preferably 0.008% or less, and further more preferably0.005% or less.

Example

Below, the present invention will be explained more specificallyreferring to an example. The present invention is not limited by theexample described below, and it is a matter of course that the presentinvention can also be implemented with modifications being addedappropriately within the scope adaptable to the purposes described aboveand below, and any of them is to be included within the technical rangeof the present invention.

After steel of the chemical composition shown in Tables 1, 2 was smeltedaccording to an ordinary smelting method, a billet of 150 mm×150 mm wasmanufactured and was heated inside a heating furnace. Thereafter, theprimary scale formed inside the heating furnace was descaled usinghigh-pressure water, hot rolling was executed under the conditions (thewinding temperature after hot rolling and the gas used for cooling)shown in Table 3, and the steel wire material of Φ5.5 mm was obtained.Also, cooling using the gas shown in Table 3 was executed toapproximately 600° C. in all cases, and the wire material was left forcooling in the atmospheric air.

The obtained steel wire material was measured by a method describedbelow.

(1) Measurement of Thickness of Scale

Samples with 10 mm length were taken from the front end, center part andrear end of the coil respectively, and the cross sections of the scaleof optional three locations from each sample were observed using ascanning electron microscope (SEM) (observation magnification: 5,000times). The scale thickness was measured for 10 points at every 100 μmlength in the peripheral direction of the steel wire material on eachmeasurement location, the average scale thickness thereof was obtained,and the average value of the three locations was made the scalethickness of each sample. Further, the average value of respectivesamples (the front end, center part and rear end of the coil) wascalculated, and was made the scale thickness of each test No.

(2) Measurement of Composition of Scale

Similarly to above (1), samples with 10 mm length were taken from thefront end, center part and rear end of the coil respectively, X-raydiffraction was performed for the cross sections of the scale ofoptional three locations from each sample, and the ratios (vol %) of FeOand Fe₂SiO₄ were obtained from the peak intensity ratio of FeO, Fe₂SiO₄,Fe₂O₃ and Fe₃O₄. The average values of the three locations were made theFeO ratio and the Fe₂SiO₄ ratio of each sample. Further, the averagevalue of respective samples (the front end, center part and rear end ofthe coil) was calculated, and was made the FeO ratio and the Fe₂SiO₄ratio of each test No.

(3) Measurement of Scale Peeling Performance of Rolled Material

Samples with 200 mm length were taken from the front end, center partand rear end of the coil respectively, air was blown to the sample, andthe scale on the surface of the steel wire material was blown out. Theappearance before and after blowing the air was photographed by adigital camera, and the area ratio of the scale having peeled off wasobtained by comparing the both by image analysis.

(3) Measurement of MD Performance

Samples with 250 mm length were taken from the front end, center partand rear end of the coil respectively, were applied with deformationstrain of 6% by a tensile test machine, and were taken out from thechuck. Air was thereafter blown to the sample, and the scale on thesurface of the steel wire material was blown out. The appearance beforeand after application of the strain was photographed by a digitalcamera, and the area ratio of the remaining scale was calculated bycomparing the both by image analysis.

The results are shown in Tables 4, 5 and FIG. 2.

TABLE 1 Chemical composition (mass %) with the remainder being iron andunavoidable impurities Steel kind C Si Mn P S N Cr Ni Cu Al B Others A-10.80 0.25 0.55 0.007 0.003 0.002 — — — — — — A-2 0.80 0.25 0.55 0.0070.003 0.002 0.28 — — — — — A-3 0.80 0.25 0.55 0.007 0.003 0.002 — 0.23 —A-4 0.80 0.25 0.55 0.007 0.003 0.002 — — 0.18 — — — A-5 0.80 0.25 0.550.007 0.003 0.002 — — — 0.025 — — A-6 0.80 0.25 0.55 0.007 0.003 0.002 —— — 0.0005 — A-7 0.80 0.25 0.55 0.007 0.003 0.002 — — — — — V = 0.035A-8 0.80 0.25 0.55 0.007 0.003 0.002 — — — — — Ca = 0.004 A-9 0.80 0.250.55 0.007 0.003 0.002 — — — — — Hf = 0.052 A-10 0.80 0.25 0.55 0.0070.003 0.002 — — — — — Ti = 0.038 A-11 0.80 0.25 0.55 0.007 0.003 0.002 —— — — — Mg = 0.003 A-12 0.80 0.25 0.55 0.007 0.003 0.002 — — — — — Nb =0.031 A-13 0.80 0.25 0.55 0.007 0.003 0.002 — — — — — Zr = 0.056 A-140.80 0.25 0.55 0.007 0.003 0.002 0.23 0.03 — — — — A-15 0.80 0.25 0.550.007 0.003 0.002 0.14 0.13 0.07 — — — A-16 0.80 0.25 0.55 0.007 0.0030.002 0.05 0.09 — 0.011 — — A-17 0.80 0.25 0.55 0.007 0.003 0.002 0.120.25 — — 0.0011 — A-18 0.80 0.25 0.55 0.007 0.003 0.002 0.08 0.08 — — —Ti = 0.072 A-19 0.80 0.25 0.55 0.007 0.003 0.002 0.05 — 0.05 — — — A-200.80 0.25 0.55 0.007 0.003 0.002 0.16 — 0.14 0.028 — — A-21 0.80 0.250.55 0.007 0.003 0.002 — 0.15 0.09 — — — A-22 0.80 0.25 0.55 0.007 0.0030.002 — 0.28 0.17 0.035 — — A-23 0.80 0.25 0.55 0.007 0.003 0.002 — 0.120.06 — 0.0009 — A-24 0.80 0.25 0.55 0.007 0.003 0.002 — 0.07 0.15 — — Hf= 0.054

TABLE 2 Chemical composition (mass %) with the remainder being iron andunavoidable impurities Steel kind C Si Mn P S N Cr Ni Cu Al B Others B0.06 0.08 0.12 0.003 0.005 0.002 0.12 0.03 — 0.01  — V = 0.029 C 0.190.17 0.42 0.003 0.001 0.002 — — 0.04 0.021 — — D 0.44 0.38 0.88 0.0020.003 0.002 — — — — — Ca = 0.002 E 0.69 0.45 0.76 0.002 0.004 0.002 —0.01 0.02 — 0.0005 Ti = 0.031, Hf = 0.027 F 0.88 0.37 0.46 0.002 0.0030.002 0.27 — 0.03 0.015 — — G 0.92 0.48 0.98 0.002 0.003 0.002 0.18 0.230.02 — — Ca = 0.003 H 1.05 0.29 1.15 0.004 0.003 0.002 0.26 0.02 0.160.002 0.0021 Ti = 0.026, Hf = 0.022 I 1.19 0.32 1.32 0.002 0.001 0.0020.03 0.14 0.12 0.001 0.0034 Zr = 0.027, Nb = 0.043

TABLE 3 Manufac- Winding turing temperature condition (° C.) Cooling gascomposition a 750 Nitrogen b 760 Atmospheric air c 800 Nitrogen + 1 vol% oxygen d 850 Nitrogen + 5 vol % oxygen e 875 Nitrogen + 10 vol %oxygen f 880 Atmospheric air g 890 Nitrogen h 740 Nitrogen i 970Atmospheric air

TABLE 4 MD performance Scale peeling Remaining scale Scale ratio ofrolled area ratio after Steel Manufacturing thickness FeO ratio Fe₂SiO₄ratio material applying 6% strain No. kind condition (μm) (vol %) (vol%) (area %) (%) 1 A-1 a 1.5 35 0.02 0.6 28 2 A-1 d 4.6 47 0.05 0.8 18 3A-1 b 1.2 26 0.01 1.9 35 4 A-2 a 1.9 36 0.03 0.8 21 5 A-3 c 2.3 39 0.010.6 18 6 A-4 d 5.9 63 0.05 0.5 10 7 A-5 e 6.8 67 0.06 0.2 8 8 A-6 c 3.252 0.01 0.6 13 9 A-7 a 1.3 41 0.02 0.5 17 10 A-8 d 4.4 52 0.01 0.8 16 11A-9 e 6.1 72 0.08 0.6 19 12 A-10 a 2.4 33 0.02 0.9 20 13 A-11 e 7.0 790.03 0.8 29 14 A-12 d 6.5 53 0.07 1.1 11 15 A-13 c 2.4 36 0.01 0.8 16 16A-14 a 1.9 34 0.01 1.2 24 17 A-15 c 2.7 39 0.02 0.9 25 18 A-16 d 4.5 480.02 0.7 12 19 A-17 e 5.3 56 0.05 0.5 5 20 A-18 c 3.4 43 0.03 0.6 11 21A-19 e 5.5 61 0.09 0.7 9 22 A-20 d 4.1 46 0.04 0.8 12 23 A-21 a 1.3 310.03 0.9 18 24 A-22 c 1.9 42 0.02 0.4 14 25 A-23 d 3.8 47 0.02 0.5 12 26A-24 e 5.4 55 0.05 1.1 7

TABLE 5 MD performance Scale peeling Remaining scale Scale ratio ofrolled area ratio after Steel Manufacturing thickness FeO ratio Fe₂SiO₄ratio material applying 6% strain No. kind condition (μm) (vol %) (vol%) (area %) (%) 27 B a 1.1 39 0.02 0.8 16 28 B c 1.8 44 0.03 1 13 29 B b1.5 21 0.02 1.8 41 30 C c 2.4 42 0.04 0.8 15 31 C d 3.6 58 0.04 0.5 1132 D e 5.5 71 0.05 0.7 22 33 D g 8.8 82 0.16 0.03 32 34 E c 2.5 32 0.030.9 27 35 E e 5.6 44 0.04 0.7 19 36 E f 6.1 25 0.04 1.9 31 37 F d 3.4 450.02 0.9 17 38 F a 1.5 31 0.01 0.8 24 39 F e 4.1 54 0.02 0.6 10 40 F b1.6 14 0.01 1.9 52 41 G a 1.5 44 0.01 1 14 42 G d 3.2 57 0.04 0.5 7 43 Gf 6.0 27 0.05 1.8 35 44 H a 0.9 31 0.01 0.7 18 45 H e 4.5 61 0.03 0.6 1046 H b 1.8 8 0.02 1.8 56 47 H f 6.7 19 0.08 1.9 43 48 I d 4.8 68 0.050.8 14 49 I h 0.8 6 0.01 1.8 59 50 A-1 i 7.1 41 0.02 2.9 10 51 B i 7.538 0.01 3.5 11 52 E i 8.2 45 0.03 5.7 9 53 G i 7.8 42 0.02 4.2 12 54 I i8.5 35 0.01 6.1 4

Nos. 1, 2, 4-28, 30-32, 34, 35, 37-39, 41, 42, 44, 45, 48 of Tables 4, 5are examples satisfying the requirements of the present invention, thescale thickness and the composition of the scale are appropriate, andtherefore the MD property is excellent.

On the other hand, in Nos. 3, 29, 33, 36, 40, 43, 46, 47, 49, the MDproperty deteriorated, because the manufacturing condition did notsatisfy the requirements of the present invention.

Nos. 3, 29, 36, 40, 43, 46, 47 are examples cooling was executed byinjecting the atmospheric air after hot rolling, the FeO fraction couldnot be secured because FeO was converted to Fe₃O₄ during cooling, andthe MD property deteriorated. No. 33 is an example the windingtemperature after hot rolling was high, the scale thickness becamethick, the FeO ratio increased excessively, the Fe₂SiO₄ ratio was alsohigh, and therefore the MD property deteriorated. No. 49 is an examplethe winding temperature after hot rolling was low, the FeO ratio couldnot be secured, and the MD property deteriorated. Nos. 50-54 areexamples the winding temperature after hot rolling was further high, thescale thickness exceeded 7.0 μm, the scale peeling ratio of the rolledmaterial increased, and the rust was generated. More specifically, inNos. 50-54, it is considered that the scale drops during cooling afterhot rolling and at the time of storage and transportation, and the rustis generated.

Also, the relation between the scale thickness and the scale peelingratio of the rolled material is shown in FIG. 2. It is known that thescale peeling ratio of the rolled material increases when the scalethickness becomes thick exceeding 7.0 μm.

The present invention has been described in detail and referring to aspecific embodiment, however, it is clear for a person with an ordinaryskill in the art that a variety of alterations and modifications can beadded without departing from the spirit and scope of the presentinvention.

The present application is based on the Japanese Patent Application No.2011-002014 applied on Jan. 7, 2011, and the contents thereof are herebyincorporated by reference.

INDUSTRIAL APPLICABILITY

The steel wire material of the present invention is excellent in themechanical descaling performance after hot rolling (before drawingwork), and is therefore useful as a raw material for a tire cord (steelcord, bead wire) for an automobile, hose wire, a saw wire and the likeused for cutting a silicon for a semiconductor and the like.

1. A steel wire material comprising, by mass % based on the total massof the steel wire material: C: 0.05-1.2%; Si: 0.01-0.5%; Mn: 0.1-1.5%;P: 0.02% or less (not including 0%); S: 0.02% or less (not including0%); and N: 0.005% or less (not including 0%); with the remainder beingiron and unavoidable impurities, wherein a scale with 7.0 μm or lessthickness is included, FeO ratio inside the scale is 30-80 vol %, andFe₂SiO₄ ratio is less than 0.1 vol %.
 2. The steel wire materialaccording to claim 1, further comprising, by mass % based on the totalmass of the steel wire material, at least one selected from the group of(1) (2), (3), (4), (5), and (6): (1) Cr: 0.3% or less (not including 0%)and/or Ni: 0.3% or less (not including 0%); (2) Cu: 0.2% or less (notincluding 0%); (3) At least one element selected from a group consistingof Nb, V, Ti, Hf and Zr by 0.1% or less (not including 0%) in total; (4)Al: 0.1% or less (not including 0%); (5) B: 0.005% or less (notincluding 0%); and (6) Ca: 0.01% or less (not including 0%) and/or Mg:0.01% or less (not including 0%).
 3. A method for manufacturing a steelwire material, the method comprising: hot rolling steel of the chemicalcomposition according to claim 1, to obtain a hot rolled steel;thereafter winding up the hot rolled steel at 750-880° C., to obtain awound steel; and cooling the wound steel while injecting a gas mixtureof oxygen and an inert gas whose oxygen fraction is less than 20 vol %or an inert gas.
 4. The method for manufacturing according to claim 3,wherein the inert gas is nitrogen.